Apparatus for dissipating wave energy

ABSTRACT

Described is a power-dissipating termination for wave energy transmission lines, characterized in having a low voltage standing wave ratio essentially independent of input power and operable in ambient temperatures up to 300* C. over a frequency range from zero to ultrahigh frequencies and above.

United States Patent Inventors Noel C. Peterson Severna Park, Md.;Gerald I. Klein, Westbury, N.Y. Appl. No. 1,353 Filed Jan. 8. i970Patented Nov. 16, 1971 Westinghouse Electric Corporation Pittsburgh, Pa.

Assignee APPARATUS FOR DISSIPATING WAVE ENERGY 8 Claims, 4 Drawing Figs.

US. Cl 333/22 R, 333/81 A Int. Cl HOlp 1/26 Field of Search 333/22, 81,84

References Cited UNITED STATES PATENTS 1/1967 Maines... 333/22 10/1967Barker....

11/1967 Steidlitz OTHER REFERENCES Blackburn, Components Handbook,"McGraw-Hill, New York, 1949, TK453B5, title pg. and pp. 70 72 PrimaryExaminer-Herman Karl Saalbach Assistant Examiner-Marvin NussbaumAt!orneysF. H. Henson and E. P. Klipfel ABSTRACT: Described is apower-dissipating termination for wave energy transmission lines,characterized in having a low voltage standing wave ratio essentiallyindependent of input power and operable in ambient temperatures up to300 C, over a frequency range from zero to ultrahigh frequencies andabove.

PAIENTEnuuv 16 ran FIG. 4.

IN VEN TORS. NOEL C. PETERSON 8 GERALD I. KLEIN B (Te 4 A r rorneyAPPARATUS FOR DISSIPATING WAVE ENERGY BACKGROUND OF THE INVENTION Incertain applications, it becomes necessary to terminate a wave energytransmission line in a load which will dissipate varying amountsofenergy over a wide frequency range. One of the most importantrequirements of such a termination is that it present a low voltagestanding wave ratio over a wide temperature range, meaning that theimpedance of the termination must remain essentially constant over therange of temperatures. Furthermore, the device must be capable oftransferring heat generated in a resistive load to its surroundingsrapidly and efiiciently. These requirements become especially difficultwhen the physical dimensions of the device are limited, or when it mustoperate at high altitudes where there is little surrounding atmosphere.

Most known devices for dissipating electrical wave energy areunsatisfactory if the foregoing conditions are to be met. For example, ahot wire (as in an incandescent bulb) will not operate as a load unlessthe applied power is essentially constant. At very low powers, the coldwire acts like a dead short. Consequently, such devices cannot be usedwhere, for example, the device must dissipate power from zero up to 25watts. Another energy dissipating device such as a long helical wirecould be designed to match 50 ohms at 250 megahertz, but it is notpossible to obtain the necessary attenuation for matching and still keepwithin reasonable dimensions. A good termination can be obtained for lowpower, room temperature operation by using a carbon resistor, but itsvalue will vary too widely to provide a constant low voltage standingwave ratio over a range of temperatures.

Liquid dielectric loads, which can handle large powers, are unsuitableif the ambient temperature is high; and certain loads require the needfor forced cooling, thereby complicating the device and increasing itssize. Dry loads for wave energy transmission lines are generally ruggedand can mount at any position. They usually consist of either acylindrical film resistor on a dielectric rod as a section of the centerconductor of a coaxial transmission line, or a disc-shaped resistorconnecting the inner and outer conductors of the transmission line. Thepower absorbed by such devices is limited by the poor heat flow paththrough small cross sections of the dielectric materials and the abilityof the surrounding air to take heat from the outer conductor. Otherloads utilizing materials which depend upon magnetic loss mechanisms areineffective in the 200 to 400 megahertz range. Most of these materialsare also temperature sensitive and/or use organic binders which areunserviceable at higher temperatures.

SUMMARY OF THE INVENTION As an overall object, the present inventionseeks to provide a new and improved radiofrequency termination forelectromagnetic wave energy transmission lines.

More specifically, an object of the invention is to provide a new andimproved wave guide termination that offers a low voltage standing waveratio independent of input power up to tens of watts and ambienttemperatures up to 300 C. over a frequency range from zero to ultrahighfrequencies and above.

In accordance with the invention, apparatus for dissipating wave energypassing through a transmission line is provided including means forcausing the wave energy to propagate in the space between a pair ofelectrically insulated conductors. Preferably, this means comprises aconventional coaxial transmission line. A resistive element is connectedbetween the center and outer conductors of the coaxial line and isformed from a material whose resistivity remains essentially constantover a wide temperature range. Preferably, this material comprisesNichrome (Trademark) comprising an alloy of 80 percent nickel andpercent chromium. I-IOwever, as a substitute, Constantan (Trademark), analloy of 55 percent copper and 45 percent nickel, or Manganin(Trademark), an alloy of 13 percent manganese and 87 percent copper canbe used.

The resistive element, preferably vapor deposited on an aluminum oxidesubstrate in the form of a film, is sandwiched between dielectric slabswhich are usually formed from boron nitride but may also be formed fromberyllium oxide. The main requirements of the dielectric material arethat it have high thermal conductivity and low electrical conductivity.Finally, the resistive element between the dielectric slabs issurrounded by a heat sink formed from upper and lower aluminum slabsseparated by a spacer formed from a metal whose coefficient of thermalexpansion closely matches that of the dielectric slabs. Means areprovided to maintain the upper and lower aluminum slabs in closeabutting and good thermal contact with the dielectric slabs, wherebyminimized thermal barriers occur at the dielectric-to-metal interface.

The above and other objects and features of the invention will becomeapparent from the following detailed description taken in connectionwith the accompanying drawings which form a part of this specification,and in which:

FIG. 1 is an exploded view of the resistive termination of theinvention;

FIG. 2 is a cross-sectional view of the termination of the invention inits assembled form;

FIG. 3 is an illustration of an alternative embodiment of the filmresistor of the invention wherein the resistor assumes a serpentineconfiguration; and

FIG. 4 is an illustration of still another embodiment of the inventionemploying stacked layers of films of resistive materia].

With reference now to the drawings, and particularly to FIG. 1, an endconnector 10 for a coaxial line is shown which receives a centerconductor 12 having a flattened area 14 at one end thereof. Theflattened end 14,. in turn, engages a contact 15 of conductive materialdeposited on an aluminum oxide substrate 16, perhaps best shown in FIG.2. Also deposited on the aluminum oxide substrate 26, by vapordeposition techniques, is a film resistor 18, the opposite end of theresistor 18 being engaged by a second vapor-deposited contact 20.

The aluminum oxide substrate 16 with the film resistor 18 depositedthereon rests on a lower dielectric slab 22; while above the filmresistor 18 is a second dielectric slab 24. The slabs 22 and 24 with thealuminum oxide substrate 15 and film resistor 18 sandwiched therebetweenare received within an opening 26 in a spacer 28. On top of the spacer28 is a first aluminum block 30 and beneath the spacer 28 is a secondaluminum block 32, perhaps best shown in FIG. 2. The entire assembly isheld together by means of a plurality of screws 34. The screws 34 haveheads 36 which fit into countersinks 38 formed in the upper aluminumblock 30 and are provided with shank portions which pass throughopenings in the spacer 28 and are threaded into the lower aluminum block32.

In order to effectively dissipate wave energy over a wide range oftemperatures, the film 18 must have a resistivity which remainsessentially constant over that temperature range. For this purpose,Nichrome (Trademark), an alloy of percent nickel and 20 percentchromium, is desired. It has a high resistivity, low change inresistance with temperature, good adherence to dielectric materials, andease of achieving a stable surface, protected by a layer of chromiumoxide after deposition. Although an alloy film of this type is somewhatmore difficult to deposit than a pure metal, no pure metal offers all ofthese advantages. Other alloys whose resistivity remains constant over awide temperature range can be used in place of Nichrome. These areConstantan (Trademark), an alloy of55 percent copper and 45 percentnickel or Manganin (Trademark), an alloy of l3 percent manganese and theremainder copper. Nichrome is desired, however, since its adherence andstability are better that the latter two materials.

The dielectric slabs 22 and 24 should have high thermal conductivity,low thermal expansion, a low dielectric constant, and goodmachinability. For this purpose, boron nitride is preferred; however incertain cases beryllium oxide can be used in its place.

The spacer 28 is preferably formed from Kovar (Trademark) comprising analloy of 20 percent nickel, 17 percent cobalt, 0.1 percent manganese andthe balance iron. This material, like the dielectric slabs 22 and 24,has a very low coefficient of expansion. The aluminum slabs 30 and 32,while having excellent thermal conductivity characteristics fordissipating the heat generated by the resistor 18, have high'coefficients of thermal expansion. AS shown in FIG. 2, the upperaluminum slab 20 is provided with a stepped portion 40 which protrudesdown into the opening 26 formed in the spacer 28. The expansion of thisstep is exactly the same as that of the spacers 28, so that the netexpansion of the cavity occupied by the boron nitride spacers 22 and 24is zero over the range from C. to 400 C. This technique of compensationfor thermal expansion is also used in fastening the device together. Theexpansion of the the spacer 28 and that portion of the upper aluminumslab 30 beneath the screw head 36 will exactly equal the expansion ofthe screws 23 which are formed from cold rolled steel. Thus, the jointsof the device will remain tight, without stress, up to 400 C. whilemaintaining minimized thermal barriers at the dielectric-to-metalinterfaces.

In the manufacture of the device, the film resistor 18 and its contactsare vacuum deposited. A high temperature silver paint is then applied tothe contacts to insure broad area, low resistance joints to the inputcenter conductor 14 and to the Kovar spacer 28, which is grounded. Theboron nitride element is then vacuum fired to drive the volatilematerials from V the paint. The joints between the outer conductor andspacer, outer conductor and connector, and center conductor to spacerare all hard brazed for good conductivity and mechanical strength. Adevice such as that shown in FIG. 2 has a voltage standing ratio of lessthan 1.20 throughout a wide range of operating conditions. In FIG. 3, aconfiguration is shown wherein a resistor 18A is again vacuum depositedon an aluminum oxide substrate 16A. However in this case, the resistor18A assumes a serpentine configuration to increase its length for agiven amount of area. Contacts 15A and 20A are deposited at the ends.This assembly would then be substituted for elements l5, 18, 20 and 22of FIGS. 1 and 2.

In FIG. 4, still another embodiment of the invention is shown whereinserpentine resistors 42A-42E are each disposed between slabs ofdielectric material 48 and 50, the ends of the serpentine conductorsbetween the respective layers being interconnected by means ofelectrical strip conductors 52. The entire assembly is encased within ahousing 54 which may, for example, be formed from aluminum.

Although the invention has been shown in' connection with certainspecific embodiments, it will be readily apparent to those skilled inthe art that various changes in form and arrangement of parts may bemade to suit requirements without departing from the spirit and scope ofthe invention.

We claim as our invention:

1. Apparatus for dissipating wave energy passing through a wave energytransmission line, comprising means for causing said wave energy topropagate in the space between a pair of electrically insulatedconductors, a resistive element connected between said conductors, saidresistive element being sandwiched between slabs of dielectric materialhaving a low coefficient of expansion, and a heat sink surrounding saiddielectric slabs and in contact therewith, said heat sink being formedfrom a spacer of material having an opening therein which receives saidresistive element sandwiched between said dielectric slabs, and blocksof metal secured to said spacer on opposite sides of said opening and inthermal contact with said dielectric slabs.

2. The apparatus of claim 1 wherein said resistive element comprises analloy comprising percent nickel and 20 percent chromium.

3. The apparatus of claim 1 wherein said resistive element is formedfrom an alloy comprising 55 percent copper and 45 percent nickel.

4. The apparatus of claim 1 wherein said dielectric material com risesboron nitride. I

5. he apparatus of claim 1 wherein said dielectric material comprisesberyllium oxide.

6. The apparatus of claim 1 wherein said spacer is formed from a metalhaving essentially the same coefficient of thermal expansion as saiddielectric slabs and said blocks are formed from aluminum.

7. The apparatus of claim 6 wherein said spacer and said blocks aresecured together by screws which pass through one of said blocks andsaid spacer and are threaded into the other of said blocks, one of saidblocks having a portion which extends into the opening in said spacerand abuts one of said dielectric slabs, the expansion of said portionupon heating along the depth of said opening being essentially equal tothe expansion of the spacer itself along the depth of said opening.

8. The apparatus of claim 7 wherein said screws are provided with headswhich abut said one block, the axial expansion of said screws uponheating being essentially equal to the expansion of said spacer and thematerial of said one block beneath said heads along the axes of saidscrews.

1. Apparatus for dissipating wave energy passing through a wave energytransmission line, comprising means for causing said wave energy topropagate in the space between a pair of electrically insulatedconductors, a resistive element connected between said conductors, saidresistive element being sandwiched between slabs of dielectric materialhaving a low coefficient of expansion, and a heat sink surrounding saiddielectric slabs and in contact therewith, said heat sink being formedfrom a spacer of material having an opening therein which receives saidresistive element sandwiched between said dielectric slabs, and blocksof metal secured to said spacer on opposite sides of said opening and inthermal contact with said dielectric slabs.
 2. The apparatus of claim 1wherein said resistive element comprises an alloy comprising 80 percentnickel and 20 percent chromium.
 3. The apparatus of claim 1 wherein saidresistive element is formed from an alloy comprising 55 percent copperand 45 percent nickel.
 4. The apparatus of claim 1 wherein saiddielectric material comprises boron nitride.
 5. The apparatus of claim 1wherein said dielectric material comprises beryllium oxide.
 6. Theapparatus of claim 1 wherein said spacer is formed from a metal havingessentially the same coefficient of thermal expansion as said dielectricslabs and said blocks are formed from aluminum.
 7. The apparatus ofclaim 6 wherein said spacer and said blocks are secured together byscrews which pass through one of said blocks and said spacer and arethreaded into the other of said blocks, one of said blocks having aportion which extends into the opening in said spacer and abuts one ofsaid dielectric slabs, the expansion of said portion upon heating alongthe depth of said opening being essentially equal to the expansion ofthe spacer itself along the depth of said opening.
 8. The apparatus ofclaim 7 wherein said screws are provided with heads which abut said oneblock, the axial expansion of said screws upon heating being essentiallyequal to the expansion of said spacer and the material of said one blockbeneath said heads along the axes of said screws.